The present invention relates generally to Radio Frequency Identification (RFID) and more specifically to the encoding of RFID tags with information concerning a collection of items.
In a conventional supply chain, goods are transported between locations in collections such as palettes of goods. An individual item is often grouped and then re-grouped as it moves through the supply chain. Information concerning goods as they move through a supply chain can be valuable in inventory control and for detecting potential points at which goods are being lost.
RFID tags can be used to identify goods as they move through a supply chain. RFID systems allow information embedded in RFID tags to be manipulated wirelessly. RFID tags are typically categorized as being passive (i.e., having no internal power supply), active (i.e., having an internal power supply), or semi-passive (i.e., having a power source to power a microprocessor only). In supply chain management applications, at various waypoints in a supply chain, a good's RFID tag may be read as the good is re-grouped at each stage of the chain, prior to reaching its ultimate destination. When a group of goods is formed, the RFID tags affixed to the goods can be considered a population of RFID tags.
Conventional RFID systems attempt to read all RFID tags that can be energized. Laws of physics typically prevent achieving a 100% read rate of a population of RFID tags at all times. The causes of failure are often related to the propagation environment surrounding each RFID tag in the population or propagation impairments. For example, tags in the middle of a pallet may not be energized sufficiently; the orientation of the tag can prevent a successful read due to the in-lay; mortality of tags can render a tag unreadable; or high RF attenuation due to the product material (e.g., metal or liquid) can shield the tags from a reader.
A shortcoming of many conventional RFID systems is that there is no way of establishing when every RFID tag in a population of RFID tags has been read. This contrasts the case of bar code readers where operators hear “a beep” which indicates a successful tag read. There is no such mechanism available today in RFID systems. The term “verifiability” can be used to imply the ability to detect the presence of an entire population of RFID tags with or without having physically read each tag in the population.
An additional challenge of many RFID applications relates to so-called “cross-reads”. Cross reads arise when one population of tags is illuminated with energy and RFID tags from another, unintended, population respond. Since electromagnetic energy moves freely in space, there is no way to prevent wave propagation into unintended areas of interest.